This invention relates to storage tubes for the storage and subsequent extraction of information and, more specifically, to improvements in a storage tube target of the type described and claimed in Kato et al. U.S. Pat. No. 4,215,288 entitled "Storage Target for Scan Converter Tubes" and dated July 29, 1980. The invention also specifically pertains to a method of fabricating the improved storage target.
In addition to the above referenced patent the following patents have particular pertinence to the instant application: (1) Kato et al. U.S. Pat. No. 4,288,720 entitled "Method for Erasing Information in a Scan Converter Storage Tube" and dated Sept. 8, 1981; and (2) Kato et al. U.S. Pat. No. 4,262,230 entitled "Storage Target for Direct View Storage Tubes" and dated Apr. 14, 1981.
The first recited U.S. Pat. No. 4,215,288 proposes the use of a single crystal of sapphire (aluminum oxide, Al.sub.2 O.sub.3) for the storage layer or substrate of a storage target. The storage substrate has a collector electrode formed on its storage surface. The collector electrode is a very thin metal sheet or film of striped or latticed pattern, defining a plurality or multiplicity of openings to expose parts of the storage surface of the substrate. The second mentioned U.S. Pat. No. 4,288,720 teaches a method of erasing information from this type of storage target as used in a scan converter storage tube. The third mentioned U.S. Pat. No. 4,262,230 suggests an adaptation of the storage target for direct view storage tubes.
The prior art storage target with the monocrystalline sapphire substrate makes possible the introduction (writing) of information into the storage tube at high speed. For a still higher writing speed desired today, however, the storage target as heretofore constructed has necessitated an increase in the potential of the collector electrode.
The increase in the collector electrode potential is undesirable for more reasons than one. First, the device must be made capable of withstanding high voltages, thereby inevitably becoming bulky and expensive. Moreover, with the increase in the collector electrode potential for the writing mode, the difference becomes correspondingly greater between this and the collector electrode potential for the reading (extraction of the stored information) or erasing mode. This great difference requires expensive circuitry for setting the collector electrode at the required potential at the time of each transition from one operating mode to another.
The same problem has been encountered with the direct view storage tubes incorporating the storage target with the monocrystalline sapphire substrate described and claimed in the aforesaid U.S. Pat. No. 4,262,230. The objective of high speed writing should therefore be accomplished without an accompanying increase in collector potential.
The noted U.S. Pat. No. 4,215,288 refers to the relationship between the writing speed and the crystallographic orientation of the collector bearing surface of the monocrystalline sapphire substrate. Later study has proved, however, that what is more important for a higher writing speed is the relationship between the crystal axis of the storage substrate and the directionality of the pattern of the collector electrode thereon.
By the term "directionality" is meant the property of the specific pattern of the collector electrode being generally oriented in a particular direction. The collector electrode is more directional if it is in the form of parallel stripes than in the form of a lattice. The directionality of the parallel stripes is tantamount to the direction in which they extend.